Background: Idiopathic inflammatory myopathies (IIM) are a heterogeneous group of autoimmune disorders primarily affecting muscle tissue. Among the most common autoantibodies associated with IIMs is anti-histidyl-tRNA synthetase (HRS; Jo-1), which defines a clinically distinct subgroup of patients. Although the pathogenic mechanisms of these diseases are incompletely understood, murine histidyl-tRNA synthetase (mHRS) administration in mice has been shown to recapitulate the pathology observed in human myositis [1], which is associated with anti-Jo-1 serum autoantibodies. Our group has shown that disruption of muscle membrane repair via genetic knockout (of synaptotagmin VII, Syt VII) or via autoantibody binding (targeting TRIM72) causes a membranopathy that exacerbates inflammatory myositis [2]. Human myositis patients commonly harbor anti-TRIM72 antibodies [2]. but their pathogenic role is unclear.

Objectives: In this study, we used the mHRS-induced animal model of myositis to evaluate the role of membranopathy in IIM pathogenesis and to investigate the therapeutic potential of regulatory T cells (Tregs).

Methods: Wild-type (WT) C57BL/6 and synaptotagmin VII knockout (KO) mice (B6.129S1-Syt7tm1Nan/J) were injected intramuscularly (IM) with recombinant mHRS. At experimental endpoints, histopathology of muscle tissue was analyzed, serum was collected to measure autoantibody production, and leukocytes were isolated from muscle for phenotypic analysis by flow cytometry. To evaluate suppression of disease pathology, Tregs isolated from WT mice were co-administered with mHRS injections. Sarcolemmal membrane repair capacity in skeletal muscle was also assessed ex vivo by laser injury assays.

Results: HRS-induced myositis is associated with infiltration of leukocytes into the muscle with associated NFκB activation. IM injection of mHRS leads to the generation of not only anti-HRS but also anti-TRIM72 antibodies and also impairs membrane repair in HRS-treated mice as measured by laser injury assay. In mice with mutations in synaptotagmin VII, another membrane repair protein, HRS-induced myositis is exacerbated, including increased levels of autoantibodies compared to WT mice treated with mHRS. Regulatory T cells can rescue the inflammatory phenotype in muscle induced by HRS in SytVII KO mice.

Conclusions: Membranopathy is both a trigger for and a consequence of inflammatory myositis induced by HRS in mice. mHRS administration in WT mice causes myositis associated with delayed membrane repair and increased titers of autoantibodies against not only HRS but also other autoantigens known to be targeted in human myositis, including TRIM72. Conversely, in SytVII KO mice with congenital defects in membrane repair, both histological myositis and autoantibody production are increased. Application of Treg can ameliorate histopathological inflammation, and ongoing work is being done to determine whether autoantibody titers and/or membrane repair are similarly improved by Treg activity.

REFERENCES: [1] Katsumata Y et al. Species-specific immune responses generated by histidyl-tRNA synthetase immunization are associated with muscle and lung inflammation. J Autoimmun. 2007 Sep-Nov;29(2-3):174-86. doi: 10.1016/j.jaut.2007.07.005. PMID: 17826948; PMCID: PMC2639656.

[2] McElhanon KE et al. Autoantibodies targeting TRIM72 compromise membrane repair and contribute to inflammatory myopathy. J Clin Invest. 2020 Aug 3;130(8):4440-4455. doi: 10.1172/JCI131721. PMID: 32687067; PMCID: PMC7410050.

Acknowledgments: NIL.

Disclosure of Interests: None declared.